In historic news, for the first time in 40 years, the number of recorded abortions each year in the United States dropped below one million, according to a report from Guttmacher Institute, a leading sexual and reproductive health research and policy organization. In 2013 and 2014, American women got 958,700 and 926,200 abortions, respectively. A decades-long decline in abortion continued through 2014, falling to 14.6 abortions per 1,000 women between ages 15 and 44, which is the lowest rate on record.

The report, published online in Perspectives on Sexual and Reproductive Health, is based on the Guttmacher Institute’s 17th census of all known abortion-providing facilities in the U.S., and includes data going back to 1973, the year Roe V. Wade took effect, legalizing abortion nationwide.

The study authors didn't do a technical analysis to determine the biggest factors behind the drop, but they have a few ideas. Most encouragingly, they point to more people using birth control—and using it correctly—which has contributed to a decline in unintended pregnancies in the U.S. In 2008, 54 percent of all pregnancies were unintended; that number fell to 45 percent in 2011, according to the most recent data available. Gynecologists and reproductive rights supporters contend that when women have improved access to contraception (such as LARCs), as well as to education, they can wait to get pregnant until they're ready, reducing the need for abortion.

Another element likely lies in the extensive abortion restrictions many states have enacted. The report didn't draw a direct relationship between state restrictions and abortion rate fluctuations, but found that "six of the 10 states that experienced the greatest proportionate declines in the number of clinics enacted three or four new abortion restrictions between 2012 and 2014, and four of these states implemented new TRAP laws.”

The authors do not believe that laws and clinic closures alone are responsible for the drop in rates. They point out that two of the 10 states with the biggest declines did not enact new restrictions during the study period. The report also didn’t find a correlation between the availability of clinics and abortion rates: In the Midwest, clinics decreased by 22 percent, and the abortion rate fell by 9 percent, while in the Northeast, the number of clinics increased by 14 percent, and the abortion rate fell 11 percent. Overall, there were 6 percent fewer clinics offering abortion services across the U.S. in 2014 than in 2011.

That said, the researchers predict that if other state-level abortion-restricting laws go through (laws like the 20-week abortion ban that passed in Kentucky this month), the trend of fewer abortions will continue. “Restricting access to abortion may force women to delay the procedure or carry unwanted pregnancies to term,” Megan Donovan, Guttmacher senior policy manager, said in a statement.

“Abortion restrictions and clinic closures mean that patients may need to travel greater distances to access services,” Rachel Jones, lead author of the study, told Guttmacher. “The majority of abortion patients—75 percent—are poor or low-income, and nearly two-thirds are already parents. It can be very difficult for them to arrange for time off from work, transportation, and child care. While many find ways to access care despite these obstacles, some of the abortion rate decline is likely attributable to women who were prevented from accessing needed services.”

Although abortion is on a general downswing, the number of medication abortions, aka terminating a pregnancy using the abortion pill, is on the rise, jumping from fewer than a quarter of abortions in 2011 to nearly a third in 2014. Many people prefer medication abortions because the procedure happens at home instead of at a clinic, because they want to avoid surgery, or because it feels more natural since it's the same process as a miscarriage.

Donovan believes that the best way to continue the current trend is to bolster efforts to give women access to birth control, which reliably reduces the rates at which women seek abortions. “We should focus on increasing access to the full range of contraceptive methods, as well as to abortion services," she said. "Empowering women to prevent unintended pregnancies and plan their families is both a human rights priority and smart public health policy.”